Light emitting diodes (LEDs) can serve as a source of lighting and are typically used in applications such as outdoor full colour displays, traffic lights, data storage, solid state lighting and communications. Typically, a III-nitride based LED emits light with a wavelength corresponding to the bandgap of active Indium Gallium Nitride/Gallium Nitride (InGaN/GaN) multiple quantum well (MQW) layers. Light is generated when a bias is applied across a p-n junction of the GaN layers. In the LED, holes and electrons injected by the p-type and n-GaN layers combine at the active layers to emit light from the LED surface.
Epitaxial growth of InGaN/GaN MQWs is practically challenging e.g. when high In content is incorporated for long wavelength applications such as for green or yellow LEDs. Furthermore, light output efficiency is typically lowered for light emission with increasing wavelengths with higher In incorporation. Lowering of the growth temperature can result in an increase in incorporation of In but the lower temperature typically causes a reduction in the photoluminescence (PL) intensity as the crystalline quality is degraded.
InGaN quantum dots have been explored for enhancing quantum efficiency in LEDs [see e.g. CHOI et al., WO-2004/054006 A1]. Emission colour from LEDs can been tuned from blue to orange by controlling the dimension of quantum dots [see e.g. Grandjean et al. U.S. Pat. No. 6,445,009 B1]. DenBaars et. al in US2006/0255347 A1 describe doping GaN with one or more rare earth transitional elements to obtain a tunable LED with a variety of color shades. These transitional elements include Cr, Ti and Co and it is proposed that white light can be generated using a combination of these elements. However, it is practically difficult to control the depth of implantation into a thin active layer of an MQW structure.
Chua et al. in US2004/0023427 A1 describe white light emission using InGaN quantum dots incorporated in a well layer in MQW structures for LEDs. However, out-diffusion of the quantum dots into the well, or into a barrier layer when high temperature in-situ annealing is carried out for activation of a p-dopant Magnesium (Mg) was experienced. This typically causes the MQWs layer to be less well-defined. Thus, the confinement effect due to the quantum dots can be compromised. Furthermore, in US2004/0023427 A1, the colours of emission from the LED cannot be precisely controlled causing the degree of whiteness of the LED to vary.
Currently, visible red-orange and yellow light sources are typically achieved based on Aluminum Indium Gallium Phosphate (AlInGaP) material, while bright green, blue and violet LEDs are typically fabricated from GaN based material systems. However, in current technologies, GaN based LEDs are not known to have red emission, which is typically needed to provide a red component to generate white light emission.
Typically, to produce white light from LEDs, a combination of separate LEDs each emitting individual colours of red, green and blue light is used. Alternatively, individual blue and yellow LEDs are combined to produce white light. One disadvantage is that multiple LEDs of different materials are used to generate the white light and this increases complexity of the fabrication technique and the overall fabrication cost. The resulting device typically uses complicated control electronics since different diode types use different applied voltages. Furthermore, different degradation rates of the materials used, e.g., AlInGaP (for red emission) and InGaN (for green and blue emission), typically affect reliability or quality of the white light obtained. The above reliability/quality problem also arises for an alternative fabrication method using yellow phosphor coated blue LEDs for white light generation because of degradation of phosphor. In addition, the use of phosphor typically increases the production cost and lowers external quantum efficiency due to absorption in the phosphor.
Hence, there exists a need for a multiple quantum well structure (MQW) for a light emitting diode and a method for fabricating a MQW structure for a light emitting diode to address at least one of the above problems.